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  1. #1
    Join Date
    Feb 2008
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    30

    tankless questions

    My NG tank WH is on its last legs and I am interested in going tankless. But I have a couple of possible issues.

    First is that my 3 level townhome, with utility room on otherwise finished lower level presents limited venting options. The original WH is vented in the same type B stack as the NG furnace so that is not an option. My only option looks to be to take the Cat III stainless vent out the front side of my house on a piece of wall above my hose bib. It looks like a good place and meets all the required clearances except it I won't be able to maintain greater than 12" from an inside corner. It'll be more like 8", but it is brick. So irrespective of a possible code conflict is there any *practical* problem with this?

    Second, the unit I am looking at is 199,000 btuh. My gas supply line is 1/2" diameter 2 psig to the manifold and regulator. The manifold is 1" iron pipe with 5 outlets to furnace, range, gas fireplace, gas grill, and tank water heater. Tank WH is 40,000, range/oven is 60,000, furnace is 80,000, gas grill is 40,000, gas fireplace is 30,000.

    Is this gas supply likely sufficient to run the tankless WH and the furnace at the same time? I would run 10' of 3/4" supply line to the tankless.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,069
    Too close to inside corner, and the intake could draw combusted fumes back in, and cause burner trouble.
    Since you didn't mention how long the 1/2" pipe run is from the meter to the reg, or how many ells are in the run. Can't tell.

    Your contractor can wii figure if the gas line can handle the extra load.
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  3. #3
    Join Date
    Feb 2008
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    30
    Intake short circuit is not an issue as the tankless will draw air from inside as does the furnace. Utility room has lots of volume and is joined to the lower level by a vented door. WH won't run enough to justify doing a sealed combustion unit with outside intake.

    1/2" 2 psig line is about 25' of flex copper tubing. So no true els but maybe 2 bends.

    I don't want to get a contractor involved until I am pretty sure this is doable. That's why I am asking here. It's all about economics. No sense in paying for all sorts of evaluations and estimates which would negate any possible economic benefit, especially it the corner issue is a hard and fast code problem. Also, with finished lower level there is no economically practical way to run a larger gas supply line. Might as well just go to Home Depot and get another tank WH.

  4. #4
    Join Date
    Feb 2007
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    18951
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    1,593
    Before you make a firm decision, read as much as you can about tankless. Lots of people don't like them, and how you use hot water will probably change.

    That's a pretty big jump in btus. The guy you hire should have a book telling him gas flow rates. Pressures must be maintained, or someone might not wake up the next morning.

  5. #5
    Join Date
    Feb 2008
    Posts
    30
    I have read up a lot on tankless. Most every day my total hot water usage consists of a couple of morning showers. No sense in keeping 50 gallons cooking 24/7.

    It isn't THAT much of an increase. All the existing non-furnace loads total to about 140,000. So assuming my system is sized to run all that at the same time as the furnace, perhaps the tankless can run at the same time as the furnace, without all that other stuff running, which never happens anyway.



    Quote Originally Posted by bobb25 View Post
    Before you make a firm decision, read as much as you can about tankless. Lots of people don't like them, and how you use hot water will probably change.

    That's a pretty big jump in btus. The guy you hire should have a book telling him gas flow rates. Pressures must be maintained, or someone might not wake up the next morning.

  6. #6
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,559
    Quote Originally Posted by raylo View Post
    Intake short circuit is not an issue as the tankless will draw air from inside as does the furnace. Utility room has lots of volume and is joined to the lower level by a vented door. WH won't run enough to justify doing a sealed combustion unit with outside intake.

    1/2" 2 psig line is about 25' of flex copper tubing. So no true els but maybe 2 bends.

    I don't want to get a contractor involved until I am pretty sure this is doable. That's why I am asking here. It's all about economics. No sense in paying for all sorts of evaluations and estimates which would negate any possible economic benefit, especially it the corner issue is a hard and fast code problem. Also, with finished lower level there is no economically practical way to run a larger gas supply line. Might as well just go to Home Depot and get another tank WH.
    Most contractors (at least we will) come out if somebody is looking to change out equipment and look at it to give a quote for free. They won't be doing the work at that time, just taking a peek to see what you need.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,069
    The estimate is, should be free.

    Only a contractor that looks at the set up can tell you for sure if it is all doable.
    The tankless water heater will have run as long as you are using hot water.
    20 minute shower, means 20 minutes that the tankless burner will be on. Its a small water heater so you probaly won't take too many long showers.
    It will use a lot more combustion air then you think.

    With both the tankless and furnace in the same room, you need 13,950 CF of open space not to need combustion air from another sourse. Your vented door may not have enough free area.
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  8. #8
    Join Date
    Feb 2008
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    30
    I'll have someone out to look soon.

    The util room is 1000 CF itself and the door is vented its entire surface and communicates with the entire house with only the bathrooms and bedrooms on the 3rd level having solid doors. So plenty of volume. But sealed combustion is still a possibility. There are places an intake could be run that wouldn't be a short circuit issue.

    The only real venting problem is the exhaust since you need to maintain clearances and have room for the wall nipple (I have read several tankless manuals online). I have only one small width of util room wall that would work... except for that inside corner issue. Otherwise I'd have to get the exhaust pipe run through a bricked in space under the stairs that I have no access to. Back to that economic issue again. Much cheaper just to go with another tank.


    Who on earth takes 20 minute showers??? ;-)

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,069
    Lol.
    teenagers.
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  10. #10
    Join Date
    Aug 2007
    Location
    MN
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    2,677
    I set up a tankless for a guy last week who installed a Bosch(DIY) hadn't ran right for a yr. They need a combustion test to set up CO2 to proper levels. Just cause the factory sets them up doesn't mean they are right, this one was LP and had co2 readings that would be for Natural, adjusted the air screw and it worked great. He added a 6 gallon tank to the system cause when he washes dishes, the trickle for rinsing isn't enough to turn on the burner.

    BTW I pulled the intake to check for blockage, and it was extremely loud compared to when it was on, so I would consider having a PVC intake installed also. find an open outside wall to mount the unit on cause the stainless piping is expensive.
    You can't fix stupid

  11. #11
    Join Date
    Jan 2008
    Posts
    89
    Generally the one big usage benefit to tankless is for continuous water use. The economic benefit is tough to nail down. While you're only using water for a short period each day, a decent WH with r16 or better wont take much gas to maintain the water temp.

    Given the cost differential, for a couple of showers a day it'll take you about 20 years to get your money back on the tankless install.

    Noisy, lots of supply air and venting issues, higher unit cost, higher install cost, sometimes higher maintenance cost. But if you need to take 5-6 showers in a row and dont have room for a big tank unit or you need to put a WH in a very small area, they're pretty handy.

  12. #12
    Join Date
    Feb 2008
    Posts
    30
    Yes, the venting issues alone in my case will probably make it not worth doing tankless. Even if it's possible the install would probably double the cost.... while I could easily just hook up a new tank myself. But is it an interesting concept and would probably be more worth doing if the house design was planned for it.

  13. #13
    Join Date
    Jan 2008
    Posts
    89
    Yeah, I like the idea of it too, and did install one once. It worked fine and for the large family that bought the house, it was a nice selling point. But it was in an outside closet on the back of the house with its own roof, three huge vents in the wall, etc.

    You'd be looking at easily 2x the cost...perhaps a bit more than that. If you have the need to run more than one h/w outlet at a time (like showering and doing laundry) you might need to bump the tankless up to a medium size unit. The cost for the larger tankless units without labor is usually more than a 40-50 gallon tank unit WITH labor.

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